Field of the Invention
The present invention relates to a toner for developing electrostatic images. In particular, the present invention relates to a toner for developing electrostatic images, the toner having superior fixing properties at low temperatures and maintaining the fixing properties even after being stored at high temperatures.
Description of the Related Art
Recent electrophotographic imaging apparatuses require toners for developing electrostatic images (hereinafter may be referred to simply as “toner”) having superior low-temperature fixing properties in view of high printing rate and further energy saving for a reduction in environmental load. Such a toner requires a reduction in melting temperature or melting viscosity of a binder resin contained in the toner. Several documents propose toners containing crystalline resins (e.g., crystalline polyester resins) as fixing aids and thus having improved low-temperature fixing properties (refer to, for example, Japanese Unexamined Patent Application Publication Nos. 2001-222138 and 2014-035506).
Unfortunately, the crystalline resin-containing toner disclosed in Japanese Unexamined Patent Application Publication No. 2001-222138 may cause a problem in that high compatibility between a crystalline resin component and an amorphous resin component contained in the binder resin promotes plasticization of the binder resin before thermal fixation, resulting in poor thermal resistance during storage of the toner. In contrast, low compatibility between the crystalline resin component and the amorphous resin component may lead to insufficient plasticization of the binder resin during thermal fixation. This causes insufficient low-temperature fixing properties of the toner and poor charging characteristics of the toner due to exposure of a free crystalline resin component at the surfaces of toner particles, resulting in low density and fogging of the images.
In the crystalline resin-containing toner disclosed in Japanese Unexamined Patent Application Publication No. 2014-035506, the dispersion of the crystalline resin component in toner particles is controlled through introduction of a structure having high affinity with the crystalline resin component into an amorphous resin component in the binder resin. Introduction of this structure prevents exposure of the crystalline resin component at the surfaces of the toner particles and provides the toner with superior fixing properties, thermal resistance during storage, and charging characteristics.
Unfortunately, the fixing properties of the crystalline resin-containing toner may be impaired due to accelerated crystallization of the crystalline polyester resin during transportation of the toner in high-temperature environments (e.g., 40 to 50° C.) or during storage of the toner in an image-forming apparatus at such high temperatures. Thus, a demand has arisen for a toner having superior low-temperature fixing properties and maintaining the fixing properties even after being stored at high temperature.